1. Field of the Invention
The present invention relates generally to antenna systems, and particularly to array antennas as used in radars, communications or radiometry.
2. Technical Background
The term radar is an acronym that stands for “radio detection and ranging.” A radar system transmits radio frequency (RF) signals in a predetermined direction (i.e., a bearing or angle-of-arrival) with the intention of contacting or illuminating moving objects (“contacts”). When the transmitted radar signal illuminates a contact, a return signal is reflected back toward the radar receiver. The return signal is detected if the return signal is stronger than any noise signals that may be present in the receiver. A contact's bearing corresponds to the direction of the transmitted radar signal because the signal travels at the speed of light. The distance, or “range,” is determined by measuring the time between signal transmission and the reception of the return signal. Thus, radar systems are commonly used in commercial and military settings for purposes of identifying and tracking a radar contacts within a predetermined search volume. The radar systems described in the present invention are directed toward phased array radar systems that determine the angular direction, i.e., the angle-of-arrival, of a target relative to the phased array antenna.
A monopulse radar system is a type of radar that is often used for surveillance and tracking radar systems. Monopulse radar systems are often implemented using phased array antennas and digital beamforming processors. The term monopulse refers to the fact that a monopulse radar transmits a single radar pulse during transmission. If there is a radar target in the search volume, the transmitted signal will be reflected by the target; and the return echo is used to measure the angle of the target. Specifically, when a phased array antenna includes N antenna elements (N being an integer value), the N antenna elements will provide N signals s(1), . . . , s(N), respectively, to the receiver's beamforming processor. The beamforming network divides each of the N input signals s(n) to create two signal paths that are linearly weighted and combined to provide a sum (Σ) beam and a difference (Δ) beam. The amplitude of the sum beam (Σ) is symmetrical, with its maximum at the boresight. The amplitude of the difference beam (Δ) is antisymmetrical and is equal to zero at the boresight. The angular direction of a target with respect to the antenna boresight is determined by finding the “monopulse ratio” for the expression:Δ(θ)/Σ(θ),Which is typically a function of the array element spacing, N the number of sensor elements, and λ is the wavelength of the transmitted radar pulse. The angular direction is, of course, estimated angle (θ) of the target. Once the monopulse ratio is determined, the estimated angle (θ) is retrieved from a look-up table in memory. Before the system is put in the field, however, the antenna must be measured and calibrated such that the angular direction (θ) values are accurate.
Calibration therefore refers to accurately relating the receive antenna beam-port complex voltages of a radio frequency (RF) sensor, such as a radar, to the corresponding angular directions of a detected target. Calibration also maximizes mainbeam gain and lowers the sidelobes. While radar systems are specifically addressed herein, the present invention applies equally well to any multiport RF sensors designed to determine angle of arrival of incident waves, such as radio astronomy, communications or radiometry.
Antenna arrays radiate electromagnetic (EM) fields in response to voltage excitations at beamformer ports of the antenna or, equivalently, at element ports. The elements refer to the individual radiators in the array. Conversely, EM fields impinging on antenna arrays excite voltages at the beamformer and element ports. For many antenna applications, it is important to create tables that accurately relate the voltages to the corresponding radiated EM field patterns (in transmit; the patterns relate the field values to direction) or to the directions of incident fields (in receive). Antenna calibration is the means of ensuring that these tables are accurate.
FIG. 1 is a diagram of a conventional system that uses an internal calibration coupler feed. Those of ordinary skill in the art will appreciate that calibration couplers can be employed for both transmit and receive calibration. A calibration signal from a central source is split many ways in a manifold and a nominally-equal proportion is coupled into each element channel 1 at some point behind the radiating element. The signal level at the receiver can then be adjusted accordingly to produce the desired performance characteristics for the array antenna. When using a calibration coupler 1-1, a portion of the element channel is not included in the calibration process. One drawback to this approach relates to the fact that a portion of the element channel (i.e., the antenna dipole elements and the ground plane) is not included in the calibration process. Another drawback to using internal calibration couplers relates to their physical size. The couplers 1-1 can be relatively large; and the array antenna must incorporate them into the design without sacrificing performance. Yet another drawback relates to the differences in the coupling efficiency at each element channel. The accuracy of antenna calibration is limited by the calibration couplers 1-1 themselves, and to the extent that the individual coupler outputs can be known. In other words, the couplers and the feed network are a source of error in and of themselves.
In another approach, some antenna systems are equipped with a means for sensing element channel path variations and the ability to recover the original state at which the antenna was first calibrated. These systems are fitted with a transmitter and receiver “probe” that is usually attached to an edge of the antenna. The probe periodically generates or receives a signal that is recorded in or from all antenna element channels. Variations in amplitude and phase of each element channel path are monitored and compensating attenuator and phase shifts are applied to adjust the array into coherent transmit and receive alignment. One drawback to this approach relates to the resolution of the attenuators and phase shifters needed for precise compensation. In addition, the signal to or from the probe must remain stable over time (i.e. several years) and this is difficult, if not impossible, to ensure. If the probe signal is imprecise, the compensation will also be inaccurate.
FIG. 2 is a diagram of a conventional phased array radar that uses an external probe to perform calibration. Each antenna element is connected to a beamforming summer 8 via a T/R module 7. Each T/R module includes a circulator 2 that couples the receive side and the transmit side to the antenna and at least one phase shifter element that couples the receive side and the transmit side to the summer 8. The transmit side typically includes a high power amplifier (HPA) 3 and the receive side includes a low noise amplifier (LNA) 4. The control element 9 is configured to toggle the switch 6 between transmit and receive operations as appropriate.
The calibration method places an external probe in front of the array face 1. The calibration process can be performed by the control block 9. In this scheme, the calibration process may require an appreciable amount of time (on the order of a few minutes). While full calibration may occur when the radar equipment is initialized, it may also be accomplished from time to time with the antenna stationary and dedicated to calibration. The transmit path calibration requires the sequential operation of one module at a time. The control block 9 effects the phase shifter states in each T/R module 7 by means of the control line 9-1 shown in FIG. 2. The control line 9-1 may be implemented as a bus system, whereby the phase shifter state is addressed to the appropriate T/R module 7 via an address bus. The enablement of one T/R module at a time during transmit path calibration, which is accomplished by disabling the other three modules and allowing the exciter signal to flow through only one module at a time, also avoids redundancy in the path through the T/R sub-assembly for the particular antenna element during transmit path calibration, assuming no mutual coupling between the circuits within the T/R sub-assembly.
Calibration for receive path calibration functions in much the same way as in the transmit path calibration. Under the control of the control block 9, the receive path calibration operates each T/R module sequentially and one at a time so that the phase response of each phase shifter can be isolated and adjusted separately. One of the drawbacks to this approach—and the previous approach—is that the system under calibration is not operating under actual operating conditions. As a result, for example, the conventional calibration techniques outlined above cannot account for temperature variations.
For some array antennas, the beamformer/feed network can be separated from the radiators/balun/matching network so that element patterns can be measured with respect to these convenient ports. These patterns can then be combined with limited full-array measurements to yield calibration patterns for all beams. It isn't always practical, however, to measure element voltage gain patterns. This is especially true for nonplanar arrays whereby most elements experience different array environments and thus element gain patterns must be measured for many elements. Further, antenna arrays increasingly are being constructed of integrated componentry with few or no connectors. Decoupling the radiating part of the antenna from the beamformer/feed to enable such element pattern measurements at best introduces errors attributed to the componentry needed to connect to a receiver.
In another approach, a method for calibrating a relatively small conformal array antenna has been considered. For example, the US Army's Lightweight Counter Mortar Radar (LCMR) can be calibrated using pattern measurements of individual columns. The LCMR antenna is composed of 24 such columns deployed along a cylindrical contour. Each column is composed of a vertical beamformer and a linear vertical array of eight radiators. The column pattern data is combined with measurements of an azimuth combiner to yield calibration tables. The column voltage antenna patterns are measured either directly in an anechoic chamber or by combining bench-top apparatus measurements (of a column) with a computational electromagnetic (CEM) computer code analysis. One drawback to this approach relates to the fact that it is relatively time consuming because all columns must be measured individually. Moreover, these types of methods are error prone because the azimuth combiner is measured separately from the columns. Additional errors are introduced by the bench-top measurement because a “reference array” is needed to de-embed the radiating element part of the column from the beamformer part, and the connections between the reference array and instrumentation are not identical to the connections between the column array elements and beamformer. Perhaps most importantly, once in the field, temperature variations, mechanical stresses, and so forth degrade calibration. An efficient method of calibrating a complete system in an operational environment would not suffer these drawbacks.
Many modern antenna arrays incorporate digital phase shifters and/or attenuators at the element level. Even some versions of fixed-beam antennas have, for test purposes, built-in controllable phase shifters. For calibration, well known methods have been developed for measuring the signal received from each array element and for each phase shifter state of an array antenna while the associated system (radar or communications, etc.) is in an operational environment by cycling through the phase shifter states, collecting data with a receiving and transmitting probe, and with this data estimating the complex amplitudes and phases corresponding to all elements and phase shifter states (or the one operational state, for a fixed beam antenna). The problem is that the calibration does not directly indicate how the antenna pattern may have been altered in directions other than that of the probe.
Briefly stated, conventional calibration techniques employ internal monitor feeds (most common system) or external probes. In the first approach, calibration is carried out by using monitor feeds that are coupled to corresponding antenna elements. As noted above, this approach has the following drawbacks: antenna element level effects are not taken into account, only the array normal signal is calibrated; and calibration is no more accurate than the monitor feed itself. In the second approach, an external probe is used. While this approach avoids the monitor feed errors, heretofore, it only provides antenna calibration in the direction of the probe.
What is needed is a calibration method that overcomes the drawbacks outlined above. Namely, a calibration system and method is needed that takes into account antenna element level effects, is not limited in any way by internal monitor feed limitations and provides calibration in all desired directions. In other words, a calibration method is needed that can directly indicate how the antenna pattern may have been altered in directions other than that of the probe by taking into account the far field antenna element patterns of all antenna elements for all angles-of-arrival (θ, φ) at all system frequencies.